Direct chill (DC) casting is a vertical semi-continuous casting process used for the fabrication of cylindrical billets from non-ferrous metals such as aluminium and alloys thereof. A DC metal casting apparatus is described for example in U.S. Pat. No. 4,598,763.
A DC casting system typically includes a plurality of water-cooled moulds, each having an open-ended vertical passageway through which the liquid metal flows. As the molten metal passes through the water-cooled moulds it is cooled causing the peripheral region of the metal to freeze. The mould is usually quite short (typically 75-150 mm) and as the metal emerges from the lower end of the mould it is further cooled by water jets causing the remainder of the metal to freeze, thereby forming a cylindrical billet. The lower end of the billet is supported by a starting head (or dummy block), which is lowered gradually (typically at a rate of 50-150 mm/min) by a hydraulic ram. Liquid metal is supplied continuously to the mould until the hydraulic ram reaches its bottom position. Typically, billets produced by the DC process have a diameter of 50-500 mm with a length of 4-8 meters.
A DC casting system normally has a plurality of moulds, typically allowing 2-140 billets to be formed simultaneously. The moulds are supported by a steel casting table and are fed with molten metal through a metal distribution system. There are two principle designs of DC casting system: in the first design the flow of metal is controlled by a float and in the second design the metal flows into the mould through a feeding device made of a refractory material. This second design is often called a “hot-top” casting system.
In a typical hot-top casting system the metal distribution system includes a plurality of refractory feeding devices called “cross-feeders” that contain the liquid metal and distribute it to the moulds as the billets are formed. The cross-feeders are supported on a steel casting table and distribute the liquid metal to a plurality of casting sites beneath the table. Additional refractory components are provided beneath the table to guide the flow of liquid aluminium from the cross-feeder to the casting sites. These refractory components typically include a cylindrical sleeve (called a “thimble” or “scupper”) that fits into the circular feed hole of the cross-feeder, a circular transition plate (also called a T-plate or “top ring”) that extends radially outwards from the lower end of the thimble, and a tubular cylindrical graphite casting ring (or “casting mould”) that extends downwards from the outer periphery of the transition plate. These components may for example be as described in U.S. Pat. No. 4,598,763.
In a conventional DC casting system the refractory components may be made from different refractory materials, according to their individual requirements. For example, the thimble is usually made from a relatively dense and strong refractory material, so that it has good resistance to erosion from the molten aluminium that flows through it. Typically, for example, the thimble may be made from a cement-bonded fused silica refractory, examples of which are manufactured by Pyrotek Inc. and Rex Materials Group Limited. This material typically has a density of approximately 2 g/cm3.
An example of a formula for a conventional thimble and other metal contact refractories using fused silica bonded with cement is set out below.
Material%Fused Silica80%Non-wetting additive 5%Cement 5%Water10%Total100% 
Typically, in a conventional DC casting system, a thimble will operate for about 500 casts and the calcium silicate transition plate will operate for 250 to 350 casts.